Absorption heat pumps, absorption refrigeration machines and absorption heat transformers based on emim acetate/methanol

ABSTRACT

Absorption heat pumps, absorption refrigeration machines and absorption heat transformers (referred to as apparatus for short) operated using
     A) methanol as refrigerant and   B) a composition comprising the ionic liquid 1-ethyl-3-methylimidazolium acetate (EMIM acetate for short) as absorption medium.

The present invention relates to absorption heat pumps, absorption refrigeration machines and absorption heat transformers (referred to as apparatuses for short) which are operated using

-   A) methanol as refrigerant and -   B) a composition comprising the ionic liquid     1-ethyl-3-methylimidazolium acetate (EMIM acetate for short) as     absorption medium.

Heat pumps are apparatuses in which heat is pumped from a low temperature level to a higher temperature level with introduction of heat and/or technical work. The heat of liquefaction obtained at the high temperature level is utilized, for example, for heating. On the other hand, in the case of a refrigeration machine, the cooling of a refrigerant on depressurization and vaporization is utilized to cool a refrigerant in the external cooling circuit further. Heat pumps are not only restricted to the generation of heat and cold but also make it possible to transform the heat introduced into work, electric or mechanical energy, e.g. ORC (organic rankine cycle or Kalina process).

Conventional heat pumps and refrigeration machines are based on the effects which occur on mechanical compression and liquefaction and the depressurization and vaporization of gases and liquids, respectively, in a thermodynamic cyclic process.

In the case of absorption heat pumps, absorption refrigeration machines and absorption heat transformers, thermodynamic cyclic processes are likewise exploited for the transport of heat or for cooling. However, they are operated using a working medium pair comprising a refrigerant and an absorption medium with exploitation of the temperature-dependent solubility of the refrigerant in the absorption medium.

A known working medium pair is, in particular, ammonia (refrigerant) and ammonia/water (absorption medium); another known working medium pair is water (refrigerant) and water/lithium bromide (absorption medium).

WO 2006/084262, WO 2005/113702, WO 2006/124015 and WO 2006/124776 disclose working medium pairs comprising ionic liquids, in particular imidazolium salts, as absorption media and mention, for example, water, ammonia, halogenated hydrocarbons, argon, carbon dioxide, methanol, oxygen and nitrogen as associated refrigerants. However, the concrete working medium pair EMIM acetate/methanol is not found in the prior art.

Suitable working medium pairs for absorption heat pumps, absorption refrigeration machines and absorption heat transformers have to meet, in particular, the following requirements:

-   -   they should be nontoxic and nonexplosive     -   the refrigerant should have a high enthalpy of vaporization     -   high solubility of the refrigerant in the absorption medium,         preferably no crystallization     -   a significant reduction in the vapor pressure of the refrigerant         on dissolution in the absorption medium     -   very low vapor pressure of the absorption medium     -   very good miscibility of the refrigerant and the absorption         medium     -   good thermal conductivity of the refrigerant and the absorption         medium     -   low viscosity of the absorption medium and of the mixture of the         refrigerant and the absorption medium     -   neither the refrigerant nor the absorption medium should be         corrosive     -   a very low heat of mixing in order to achieve a high efficiency.

In addition, the pressures arising or necessary in the cyclic process should be very close to atmospheric pressure, so that a very low product of apparatus volume and gauge pressure is achieved. This makes it possible for the process to be carried out using inexpensive apparatuses.

The working medium pairs comprising ionic liquids which have been found hitherto have fundamental advantages over conventional systems, e.g. ionic liquids generally have a low vapor pressure. However, there is an additional fundamental desire for improved working medium pairs which meet the combination of all the above requirements to a very high degree.

It was therefore an object of the present invention to find working medium pairs which are very suitable for use in absorption heat pumps, absorption refrigeration machines and/or absorption heat transformers.

We have accordingly found the above-defined absorption heat pumps, absorption refrigeration machines and absorption heat transformers (also referred to collectively as apparatuses for short).

The apparatuses are operated using

-   A) methanol as refrigerant and -   B) a composition comprising the ionic liquid     1-ethyl-3-methylimidazolium acetate (EMIM acetate for short) as     absorption medium.

The refrigerant and absorption medium are also referred to collectively as working medium pairs.

The absorption medium B) comprises as substantial constituent the ionic liquid EMIM acetate. EMIM acetate is a compound of the formula

where

-   R1 is an ethyl group, -   R3 is a methyl group, -   R2, R4 and R5 are each an H atom, -   n is 1 and -   X is the acetate group (H₃C—COO—).

EMIM acetate is an ionic liquid having a melting point of less than −20° C. at 1 bar and a viscosity of 93 mPa*s at 20° C., 1 bar.

The composition B) can comprise further constituents in addition to the ionic liquid; possibilities are, for example, other additives such as corrosion inhibitors or mixture components.

In particular, EMIM acetate can be used in admixture with further ionic liquids or other absorption media. Possible further ionic liquids are, in particular, those which reduce the viscosity of the mixture further or increase the absorption capacity so that higher amounts of the refrigerant can be absorbed in the mixture.

Furthermore, the composition B) can naturally also comprise impurities such as water or other compounds which, for example, can be introduced in the preparation of the ionic liquid or by recycling processes. These can be, for example, alcohols, amines, water or salts. Salts which are comprised as impurities or are added deliberately can have a corrosion-inhibiting effect.

In a preferred embodiment, the composition B) used as absorption medium comprises more than 50% by weight, in particular more than 80% by weight, particularly preferably more than 90% by weight and very particularly preferably more than 95% by weight, of EMIM acetate.

The composition B) is preferably liquid in a temperature range from −20 to 200° C., preferably from 0 to 180° C. and particularly preferably from 20 to 150° C. (at 1 bar, atmospheric pressure).

Methanol and the composition B) are miscible with one another in the entire temperature range from −20 to 200° C., in particular from −5 to 150° C.

In addition to the refrigerant methanol, further refrigerants can be used in admixture with methanol. In a preferred embodiment, more than 50% by weight, in particular more than 80% by weight, particularly preferably more than 95% by weight, of the refrigerant used is methanol. For the purposes of the present invention, very particular preference is given to using exclusively methanol as refrigerant.

Repeated absorption processes and desorption processes occur during operation of the apparatuses of the invention, so that mixtures of the refrigerant and absorption medium are then present in the apparatuses.

The absorption heat pumps, absorption refrigeration machines and heat transformers usually comprise a liquefier, an expansion element, a boiler and an absorber and are operated using the working medium pair. In the cyclic process described, the methanol is absorbed in the absorption medium and desorbed (vaporized) again.

The apparatuses are, in particular, cooling systems or storage systems. Examples of cooling systems are refrigerators, refrigeration/freezer chests, refrigerated shelves or apparatuses for cooling rooms, e.g. air conditioning units for air conditioning of buildings and/or rooms, apparatuses for coldrooms or cooled storage spaces. As storage systems, mention may be made of, for example, cold storage, ice storage or cold water storage. In general, they are nonportable apparatuses.

The apparatuses can advantageously be small and of simple construction.

To operate the apparatuses, it is possible to utilize any heat sources, e.g. it is possible to utilize solar heat or the waste heat from an engine.

Methanol and EMIM acetate are liquid and miscible in any ratios at 20° C., 1 bar (see crystallization limits in table 1).

The good interaction between methanol and EMIM acetate brings about a large reduction in vapor pressure. Table 2 shows the large reduction in vapor pressure of the working medium pair methanol/EMIM acetate compared to other working medium pairs in the weight range up to 40% by weight of methanol. Here, the reduction in vapor pressure at from 10 to 25% by weight of methanol, in particular from 15 to 25% by weight of methanol, is important in terms of use.

Only the working medium pair methanol/bis(tributylmethylammonium)sulfate (TBMASO4) at from 10 to 25% by weight of methanol achieves an approximately equally good reduction in vapor pressure.

The apparatuses have advantageous operating points. The operating point of the absorber should be at very high temperatures and low pressures, while the desorber should have an operating point at very low temperatures.

Since the absorber has to be operated at constant temperature and the heat of absorption is liberated by absorption of the refrigerant, it has to be cooled. A high absorber temperature makes it possible to use warmer cooling media (e.g. river water) or in the most favorable case even air cooling. This increases the efficiency of the machine and reduces the complexity of the overall plant since in the most favorable case countercooling of the cooling medium can be dispensed with. The pressure in the absorber is determined by the pressure in the vaporizer and thus by the cooling temperature achieved.

The lower the cooling temperature, the higher the achievable efficiency of the absorption refrigeration plant. In the most favorable case, desorption is effected by supply of waste heat, geothermal heat or solar heat.

Proceeding from the vapor pressure curves of the refrigerant at two or three different temperatures, it is possible to construct a Duhring chart. This shows the equilibrium concentration in the vaporizer, absorber, desorber and condenser in a pressure=f (−1/temperature) graph. Important operating parameters such as maximum absorber temperature and concentration differences in the refrigerant in the absorption medium in the absorber/desorber (=degasification range) can thus be read off from the graph (see also: Absorption chillers and heat pumps, K. E. Herold, 1996, CRC Press (Boca Raton, Fla.).

Operating points (temperatures) of the absorber and desorber which have been calculated in this way for various working medium pairs are listed in table 3. The lowest operating temperature of the desorber at a high operating temperature of the absorber is obtained for the working medium pair methanol/EMIM acetate.

Furthermore, the viscosities of the mixtures of methanol and EMIM acetate are low, so that good mass transfer and heat transfer are ensured and a high efficiency is made possible (see table 4).

The working medium pair of the invention is materials compatible and leads to very little if any corrosion on components of the apparatus and to very little if any decomposition of sealing rings made of plastic.

Furthermore, the working medium pair of the invention is thermally stable up to 140° C.

TABLE 1 Crystallization limits of selected ILs in methanol Crystallization limit % by weight of IL in the IL/methanol mixture at and above which crystallization Ionic liquid (IL) occurs EMIM acetate 100 Bis(ethylmethylmorpholinium) sulfate 75 TMA OAc 55 1,1,3,3-Tetramethyl-N,N-dibutylguanidinium 100 acetate Dimethylmorpholinium acetate 65 Tributylmethylammonium sulfate 90 EMIM acetate is liquid and miscible with methanol in any ratio; there is no crystallization limit.

TABLE 2 Vapor pressures at T = 40° C. for various mixtures of methanol/ionic liquid with increasing proportion by weight of methanol % by weight of MeOH  p/mbar 1-Ethyl-3-methylimidazolium acetate 0 0 (EMIM acetate) 7.1 9.0 13.2 17.0 22.9 39.0 32.2 77.0 41.3 130.0 Bis(ethylmethylmorpholinium) sulfate 0 0 25 84 29 114 33 148 39 206 1,1,3,3-Tetramethyl-N,N-dibutylguanidinium 0.0 0 acetate 14.9 58 27.6 154 44.2 298 Bis(tributylmethylammonium) sulfate 0.0 0 (TBMASO4) 11.1 10 18.4 36 25.9 57 31.0 92 36.5 127 41.2 158 Tetramethylammonium acetate 0.0 0 11.1 35 20.0 36 27.3 41 33.3 62 38.5 86 1-Ethyl-3-methylimidazolium dicyanamide 0.0 0 11.4 86 20.4 148 27.8 185 33.9 213 39.1 223 Dimethylimidazolium acetate 0.0 0 (MMIM acetate) 11.6 125 20.8 193 28.3 224 34.5 249 39.7 259 1-Ethyl-3-methylimidazolium methanesulfonate 0.0 0 11.4 72 20.4 130 27.8 178 33.9 214 39.1 238 1-Ethyl-3-methylimidazolium diethylphosphate 0 0 30 167.6

TABLE 3 Operating points for various cold water temperatures and percentages by weight of methanol in the absorber and desorber All percentages by weight are based on the mixture of methanol/ionic liquid % by weight of methanol % by weight of in the methanol in the Absorbent T_(absorber)/° C. absorber T_(desorber)/° C. desorber EMIM OAc 49 20 122 15 MMIM OAc 54 20 132 15 Bis(tributylmethylammonium) 37 25 154 20 sulfate Refrigerant methanol Cold water temperature 5° C. EMIM OAc 44 20 112 15 Refrigerant methanol Cold water temperature 0° C. EMIM OAc 60 20 138 15 Refrigerant methanol Cold water temperature 15° C. % by weight of methanol % by weight of in the methanol in the Absorbtion medium T_(absorber)/° C. absorber T_(desorber)/° C. desorber EMIM OAc 36 20 102 15 Refrigerant methanol Cold water temperature −5° C.

TABLE 4 Viscosities at 20° C. as a function of the alcohol content Viscosity at 20° C./mPa s 15% 20% 15% IL pure MeOH MeOH EtOH 20% EtOH EMIM OAc 93 13 9 34 18 EMIM (MeO)₂PO₂ 394 25 16 79 30 

1. An absorption heat pump, absorption refrigeration machine or absorption heat transformer (referred to as apparatus for short) operated using A) methanol as refrigerant and B) a composition comprising the ionic liquid 1-ethyl-3-methylimidazolium acetate (EMIM acetate for short) as absorption medium.
 2. The apparatus according to claim 1 or 2, wherein the composition B) comprises more than 90% by weight of the ionic liquid 1-ethyl-3-methylimidazolium acetate.
 3. An absorption heat pump, absorption refrigeration machine or absorption heat transformer comprising a liquefier, an expansion element, a boiler, an absorber and A) methanol as refrigerant and B) a composition comprising the ionic liquid 1-ethyl-3-methylimidazolium acetate as absorption medium.
 4. The absorption heat pump according to any of claims 1 to 3, wherein the refrigerant A) is absorbed in the composition B).
 5. The absorption refrigeration machine according to any of claims 1 to 3, wherein the refrigerant A) is absorbed in the composition B).
 6. The absorption heat transformer according to any of claims 1 to 3, wherein the refrigerant A) is absorbed in a composition B).
 7. The absorption refrigeration machine according to any of claims 1 to 3, wherein solar heat or the waste heat from an engine is utilized for operating the apparatus. 